docs:  Add proposal amendment for deleted sysvars (#28773)
docs:  Add proposal amendment for deleted sysvars (#28773)

 
coocood/tidb,pingcap/tidb,kyle-wang/tidb,kyle-wang/tidb,c4pt0r/tidb,ngaut/tidb,ngaut/tidb,coocood/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,pingcap/tidb,c4pt0r/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,c4pt0r/tidb,ngaut/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb